The present invention relates to a structure for coupling a rotating member such as a sprocket and a shaft such as a rotating shaft and, more particularly, to a structure for coupling a shaft with a rotating member held in a case.
Generally speaking, the switching of a power transmission line is accomplished by interposing a transmission mechanism between a drive shaft and a follower shaft and is exemplified by the transfer mechanism of a four wheel drive torque transfer mechanism for a vehicle, as is disclosed in Japanese Patent KOKAI Nos. 55-72420 and 60-241575 and U.S.P. No. 4,369,671. In the four wheel drive torque transfer mechanism disclosed in those specifications, the drive force given from the output shaft of the transmission assembly is distributed through a center differential to a rear wheel output shaft and a drive sprocket, or the drive force given from the transmission assembly to the rear wheel output shaft is distributed through a clutch to the drive sprocket so that it may be applied from the drive sprocket through a chain and a driven sprocket to a front wheel output shaft. In the chain transmission means used herein, for example, the driven sprocket is splined to the front wheel output shaft, which is fitted in a case through bearings, so that it is rotatably supported. Alternatively, the drive sprocket and the front wheel output shaft are integrally manufactured in advance, and a bearing is fitted on the portion of the front wheel output shaft, which projects in the axial direction from the toothed central portion, so that the front wheel output shaft is rotatably supported through that bearing. A front propeller shaft is coupled through a universal joint or the like with the front wheel output shaft which is rotatably supported in the case, as described above.
Here, in the coupling structure for coupling the front wheel output shaft and the front propeller shaft of the transmission mechanism described above according to the prior art, the spline-fitted portions of those shafts are exposed to the outside so that they naturally cannot be lubricated with the lubrication oil in the case but have lubrication which may be deteriorated by dust or the like if they are lubricated with grease. This may possibly affect the durability. In the aforementioned structure, moreover, the four wheel torque transfer mechanism is assembled by accommodating the front wheel output shaft in the case and bearing the same by the bearings, and the front propeller shaft is coupled through the universal joint with the end portion of the front wheel output shaft. In case, therefore, the dynamic balance of the front propeller shaft is to be inspected and adjusted, these inspections and adjustments must be accomplished separately, although the front propeller shaft rotates together with the front wheel output shaft. This raises a problem that the dynamic balance cannot be accurately set or adjusted because the dynamic balance in the actually mounted state is not always recognized.
In the transmission mechanism of the prior art thus far described, furthermore, a member such as a flange yoke or a yoke sleeve is fitted on the outer circumference of the portion of the front wheel output shaft, which projects from the bearing at the front side (i.e., as viewed when the transmission mechanism is mounted on the vehicle), when the universal joint for coupling the front wheel output shaft and the front propeller shaft is to be coupled with the front wheel output shaft. As a result, the front wheel output shaft is required to have not only bosses positioned at the two sides across the driven sprocket for fitting thereon two bearings but also a fitting allowance for fitting the yoke sleeve so that its total length must be increased. Especially in the aforementioned four wheel drive torque transfer mechanism for an FR (i.e., Front-engine Rear-drive) vehicle, the front propeller shaft is required to have a constant length or more, and the universal joint coupling the front wheel output shaft and the front propeller shaft cannot be disposed in front of a fixed position, in order to void making contact with an oil pan. Consequently, in the four wheel drive torque transfer mechanism for the FR vehicle, the front wheel output shaft has to be located at a more rearward position than necessary. In other words, the total length of the four wheel drive torque transfer mechanism in the structure thus far described has to be increased more than necessary, and adaptors have to be installed between some members in the four wheel drive torque transfer mechanism, which invites an increase in the weight.